This invention pertains to medical ablation procedures and particularly to selective ablation of cardiac tissue by means of high-frequency electromagnetic energy.
The heart is composed of three types of cardiac tissue; atrial muscle, ventricular muscle and specialized excitatory and conduction tissues. The atrial and ventricular muscles of the heart are normally excited in synchrony. Each cardiac cycle begins with the generation of action potentials by the sino-atrial (SA) or sinoauricular node located in the posterior wall of the right atrium. These action potentials spread through the atrial muscle by means of specialized conduction tissue, causing contraction. The action potentials do not normally spread directly from the atrial muscles to the ventricular muscles. Instead, the action potentials conducted in the atrial musculature reach the atrioventricular (AV) node and its associated fibers, which receive and delay the impulses. Potentials from the AV node are conducted to the His-Purkinje bundle. This structure carries the impulses to the ventricular musculature to cause their synchronous contraction following contraction of the atrial muscles.
Episodes of an abnormal increase in heart rate may occur, and are termed paroxysmal tachycardia. This can result from an irritable focus in the atrium, the AV node, the bundle of His, or in the ventricles. These episodes of tachycardia may be initiated and sustained by either a re-entrant mechanism, termed a "circus" movement, or may be caused by repetitive firing of an isolated ectopic focus. While these episodes of tachycardia are usually amenable to treatment by medication, under certain circumstances surgical ablation of the abnormal focus of abnormally conducting tissue may be of value in selected instances.
Catheter techniques have been used for treatment of these tachycardias. As described in the article "Catheter Ablation in Dysrhythmias" by Gillette, published in the March 1984 issue of Cardio, standard electrophysiologic techniques involving multiple catheter recording and stimulation of the heart at several sites may be used to determine the mechanism of the cardiac dysrhymia. If pharmacological testing indicates that drugs are not useful in control of the arrhythmia, a standard intracardiac electrophysiologic catheter may be introduced and positioned adjacent to specialized conduction tissue responsible for the initiation or perpetuation of the arrythmia. The location of this tissue is usually the His bundle. Therefore, by positioning the catheter so that a His bundle electrogram is recorded, the electrode recording the His potential may then be connected to the output of a DC defibrillator. This defibrillator then delivering 3 to 5 watt-seconds per kilogram (wsec/kg) may be used to ablate that portion of the bundle of His adjacent to the electrode. This procedure may be useful in the treatment of selected cardiac arrythmias. Similarly, abnormal foci elsewhere in the heart may be treated by electrical ablation.
The Gillette procedure may be disadvantageous in the atrium, because the wall of the atrium is thin. The atrium may be perforated if the applied energy is excessive for the location. The amount of heart tissue injured by the ablating energy in this method may also undesirably exceed that which is necessary to achieve the desired ablation of the bundle of His or other ectopic focus. Furthermore, catheter ablation should be limited to the right atrium and right ventricle because of the danger of embolization due to gas or debris resulting from the procedure. Also, the application of energy from a defibrillator depolarizes the heart muscles and interrupts the normal cardiac cycle. This may be disadvantageous, especially when multiple applications of energy are required.
However, a catheter ablation procedure which is capable of reducing the amount of extraneous tissue damage during ablation, thereby reducing the danger of perforation, and which may be usable on the left side of the heart is desirable.